1. Field of the Invention
The present invention relates to an automated materials handling system and method for high through-put storage and retrieval of goods.
2. The Prior Art
Automated materials handling systems are well known in the art and typically provide for storage and retrieval of palletized goods. Computer controlled automated storage systems such as that disclosed in U.S. Pat. No. 4,074,120 have recognized the value and importance of tracking goods stored so that the goods can be retrieved in accordance with a customer's order. The tracking is accomplished by identifying the goods at or near the input station and then delivering the goods to the pickup station for pickup and storage by the storage/retrieval (S/R machines).
It is also known to permit S/R machines to obtain input loads from a conveyor which transports the load in front of the storage array for pickup by the S/R machines. In accordance with this last mentioned method, palletized goods are conveyed in seriatim in front of the storage warehouse and as the goods approach a conveyor transfer pickup station, each load is detained sufficiently long at the conveyor transfer pickup station for it to be removed from the conveyor. The load is then picked up, transferred by the S/R machine to a pre-assigned storage address and there stored for later retrieval upon command.
Inasmuch as a plurality of loads are typically simultaneously conveyed along the conveyor adjacent the high rise storage, care must be taken to assure that there is sufficient space between the pallets so that the second pallet will not catch up with the first pallet while the first pallet is being detained for transfer to the pickup station for pickup by the S/R machine. For example, if the conveyor transfer time is, for example, 10 seconds, an interpallet spacing of about 15 feet is required if the conveyor rate is sixty feet per minute. One of the critical features in determining the spacing between the loads is the time necessary to transfer the load from the conveyor to the pickup location for the S/R machine. Once the minimum transfer time has been established, it has historically been possible to space each load from the other a sufficient distance that the first load has time to clear before the second load arrives.
While this prior art technique has worked acceptably on standard pallet sizes of approximately four feet square, enormous problems have resulted for pallets which are considerably larger. For example, a high rise storage system with conveyor input and output was designed for staging automobiles at the output of an automobile manufacturing assembly line. The automobiles are placed on a pallet 12 feet by 22 feet in size. The pallets were conveyed along a chain-driven conveyor at 60 feet per minute at spacings of 30 feet. Conveyor transfer cycle times were calculated to be 35 seconds and the S/R machines were constructed to have dual cycle times of 160 seconds and single cycle times of 101 seconds. Assuming five storage aisles in the high rise storage facility, given the foregoing parameters, the system should yield a total capacity of 110 to 125 loads per hour. Operated in accordance with the prior art techniques set forth above, it was discovered that the enlarged pallets frequently came into close encounter with preceding pallets engaged in transfer to the S/R machines. The close encounter resulted in a conveyor stop delaying the free flow of pallets and materially adversely affecting through-put. Actual through-put was found to be as low as 30 loads per hour. To space the loads farther apart, while potentially reducing delays due to conveyor stoppage, would clearly adversely affect the total through-put capacity of the system.
In accordance with prior art techniques, one possible solution to the problem is to provide a separate conveyor and associated drive for each separate aisle of the high rise storage. Clearly, however, separate conveyors and drives are extremely expensive and serve to further complicate the automated storage system.